Spectral “notch” filters are optical devices operative for receiving a light signal characterized by a relatively wide spectral range and selectively passing only a relatively narrower range of wavelengths within the light signal. In other words, wavelengths outside of a “transmission window” of the notch filter are reflected or blocked in some manner, while wavelengths within the transmission window are transmitted through the device.
Of particular interest are tunable spectral filters, in which the center wavelength of the transmission window can be tuned over a spectral range. The tunability of such devices enables simplification of many optical systems and introduces the opportunity to tune complete optical systems. When a tunable spectral filter is operatively coupled with a broadband light source, for example, a narrow linewidth, wavelength-agile source can be realized. In similar fashion, a highly wavelength-selective detector results from the combination of a tunable spectral filter and a broadband detector. Tunable spectral filters have found widespread use in diverse applications, such as telecommunications, medical diagnostics (e.g., spectroscopy, optical coherence tomography (OCT), etc.), fluorescence microscopy, spectral or hyperspectral imaging, and environmental sensing, among others.
While tunable spectral filters have been developed based on a variety of different optical devices, such as liquid-crystal elements, fiber Bragg gratings, acousto-optic modulators, and surface-acoustic-wave (SAW) devices, perhaps the most commonly used is the tunable Fabry-Perot (FP) cavity.
A conventional tunable FP cavity includes a pair of parallel high-reflectivity mirrors that are closely spaced to give rise to an optically resonant cavity between them. The separation between the mirrors, referred to as the cavity length of the FP cavity, dictates what wavelengths pass through the cavity and what wavelengths are reflected by the cavity. Light having wavelength, λ, will resonate back and forth between the mirrors inside the optically resonant cavity when its cavity length, L, is equal to an integer number, N, of half-wavelengths (i.e., when L=Nλ/2) and be transmitted through the cavity with low loss. At the same time, light characterized by other wavelengths will be reflected by the FP cavity. By changing the cavity length, therefore, the wavelength of light passed by the cavity can be adjusted.
Unfortunately, prior-art tunable spectral filters are often slow, have limited tuning range, cannot operate across a wide range of wavelengths, have poor spectral resolution, and/or are complex to implement in many optical systems.